Sedimentary basins - an introduction

Definition of a sedimentary basin: A sedimentary basin is an area in which sediments have accumulated during a particular time period at a significantly greater rate and to a significantly greater thickness than surrounding areas. (Compare with physiographic basin - a depression in the surface of the land or sea-floor that may or may not be infilled with sediments).

Sedimentary basins of one type or another today cover about 70% of the Earth's surface and contain sediment thicknesses that range from about a kilometer to several 10's km. Some basins are geologically young others have existed for 100's million years.

Investigations of sedimentary basins have been the result of collaboration between the different disciplines within the Earth Sciences: where geophysics, geochemistry, structural geology, stratigraphy and sedimentology have all played an important part. A major breakthrough was the plate tectonic paradigm, which allowed basins to be classified according to a unifying geodynamic theory.

The driving mechanisms of subsidence are ultimately related to processes within the relatively rigid, cooled thermal boundary layer of the Earth known as the lithosphere. The lithosphere is composed of a number of tectonic plates that are in relative motion with one another. The relative motion produces deformation concentrated along plate boundaries which are of three basic types:

• Transform boundaries

Divergent boundaries form where new oceanic lithosphere is formed and plates diverge. These occur at the mid-ocean ridges. Convergent boundaries form where plates converge. One plate is usually subducted beneath the other at a convergent plate boundary. Convergent boundaries may be of different types, depending on the types of lithosphere involved. This results in a wide diversity of basin types formed at convergent boundaries. Transform boundaries form where plates move laterally past one another. These can be complex and are associated with a variety of basin types.

Many basins form at continental margins. Continental margins are described either as passive, where the boundary between oceanic and continental lithosphere is not a plate boundary (as around most of the present day Atlantic Ocean), or active where the ocean-continent boundary is a plate boundary associated with subduction (as around most of the present day Pacific Ocean).

Using the plate tectonics paradigm, sedimentary basins have been classified principally in terms of the type of lithospheric substratum (continental, oceanic, transitional), the position with respect to a plate boundary (interplate, intraplate) and the type of plate margin (divergent, convergent, transform) closest to the basin.

Modern classification schemes of sedimentary basins generally recognise the following principal types:

Intracratonic basins form within stable continental interiors, such as in the Michigan/Illinois basins in the Middle West, the Moscow basin on the Russian platform and the Congo basin in Africa. They are typically relatively shallow, circular in shape and have long histories of relatively slow subsidence.

Continental rift basins form narrow, elongate basins within continental interiors, such as those found in East Africa. Rapid subsidence during rifting, typically lasting 10's millions of years, may be followed by a long period of slower subsidence, as for example in the North Sea basin, which was an active rift in the Mesozoic. Other well-known recent examples include the Basin and Range province in the southwestern U.S.A. and the Baikal rift in Siberia.

Passive margin basins contain some of the world's largest sediment accumulations, often more than 20 km thick, especially where they are fed by large river systems, such as in the Mississippi and Bengal fans.

Forearc, interarc and backarc basins form at active continental margins in association with subduction and the development of island arcs. Numerous examples occur around the Pacific.

Foreland basins occur in continental collision zones as elongate depressions adjacent to actively forming mountain belts, such as the Andes, Rockies, Alps and Himalayas. The development of foreland basins is intimately related to the tectonic evolution of the mountains and ceases when mountain building comes to an end.

Pull-apart basins form in areas undergoing transform, strike-slip tectonics. They are narrow, deep basins that often show extremely fast subsidence, as typified by the basins along the San Andreas fault-zone in California.